Chlamydia are obligate intracellular parasites that are capable of surviving only in a host cell. Its growth cycle is unique, and the elementary body (hereinafter referred to as EB) of Chlamydia that is morphologically outside of the cell is taken up into the cell to form a vacuole inclusion body, which is then converted to a reticulate body (hereinafter referred to as RB). RB owns a propagating capability but lacks an infecting capability and the RB that has propagated in the cell is soon converted to an EB, which by breaking the inclusion body and disrupting the cell wall, comes out of the cell. EB lacks a propagating capability but owns an infecting capability. Currently there are confirmed four kinds of Chlamydia species (C. trachomatis, C. psittaci, C. pneumoniae, and C. pecorum), among which Chlamydia pneumoniae is known to infect humans via air infection.
In recent years, Chlamydia pneumoniae has attracted widespread attention as the causative microorganism of respiratory infections such as pneumonia, bronchitis, acute upper airway inflammation and the like. According to the serological epidemiological study conducted in various parts of the world, the prevalence of the antibody against Chlamydia pneumoniae is 40 to 50% in Europe and the USA, 60% or greater in Taiwan, Panama, Iran and the like, and 50 to 60% in Japan. As the actual situations on Chlamydia pneumoniae infections become more apparent, interests in the infections are mounting.
The most sensitive serological method for diagnosis of Chlamydia infections is the indirect microimmunofluorescence test (micro-IF test) by Wang and Grayston (Trachoma and related disorders caused by Chlamydial agents, Excerpta Medica, Amsterdam, pp.273-288, 1971). However, since the test procedure of the micro-IF test is complicated, it has not been employed as a diagnostic method in the clinical laboratories. Furthermore, the standard micro-IF test requires the purified EB of Chlamydia. The micro-IF test also requires the morphological and structural integrity of the microorganism to be identified in order to carry out the immunological fluorescent reactions. Hence the morphologically or structurally altered EB or the disrupted EB cannot be used. However, since EB has an infectious capability and toxicity, the use of an intact EB as the antigen material requires a special facility which has been rendered infection-defense. Therefore, the EB treated with a fixing agent such as formaldehyde, acetone and the like is usually used as the antigen.
On the other hand, the recently developed enzyme-linked immunosorbent assay (ELISA) has an advantage that it can process a large number of samples in a simple and rapid manner. There are reports on the methods for measurement of anti-Chlamydia antibody using the ELISA, and most of the methods employ the intact EB of Chlamydia as the antigen material. Therefore, the presence of non-specific reactions is known which result from the use of an inadequately purified antigen. This is caused largely by the complex antigenicity of Chlamydia. As the antigenicity of Chlamydia, it has been believed, there are the genus-specific antigens, the species-specific antigens, and the biobar-specific antigens.
As a representative genus-specific antigen of Chlamydia, there is known lipopolysaccharide (hereinafter referred to as LPS), which has a common antigen shared by the Re mutant LPS derived from some enterobacteria.
Furthermore, as a representative species-specific or biobar-specific antigen, there is known the Major Outer Membrane Protein of Chlamydia (hereinafter referred to as MOMP), which is considered to occupy approximately 60% of the outer membrane proteins of Chlamydia. However, the presence of the genus-specific antigenicity is also known for MOMP (Collett et al., Annu. Meet. Am. Soc. Microbiol., Washington, D.C., Abstract No. D-159, 1986).
The outer membrane antigens of Chlamydia other than MOMP are mainly genus-specific antigens, but in some the species-specific antigenicity is also present. For example, Iijima et al. report, based on the results of the immunoblot assay using the EB of Chlamydia pneumoniae, that the MOMP having a molecular weight (MW) of 40K daltons of Chlamydia pneumoniae is genus-specific, and the MW 43K-dalton, 46K-dalton and the 53K-dalton proteins are species-specific, and furthermore the MW 98K dalton-proteins are probably species-specific (Y. Iijima et al., Journal of Clinical Microbiology, p.583-588 (1994)).
As hereinabove described, the antigenicity of Chlamydia is very complicated and so antigens which are common to a genus Chlamydia exhibit a significantly different antigenicity among the different species. Hence, although the methods for measuring anti-Chlamydia trachomatis antibody are known (Japanese Unexamined Patent Publication No. Hei 4-297871), the methods cannot be simply used for measuring Chlamydia pneumoniae in a species-specific manner since the antigenicities of Chlamydia pneumoniae and Chlamydia trachomatis are quite different from each other. Also, anti-Chlamydia antibodies carried by individuals infected with Chlamydia pneumoniae show a diversity corresponding to the complex antigenicity of Chlamydia. Although the pattern varies with infected individuals, the use of EB itself as the antigen may cause non-specific reactions and hence a specific and highly precise measurement using it is difficult.